Gregarines

GREGARINES, Gregarinina (from Latin grex - herd, crowd), a subclass of protozoa from the class Sporozoa, parasites of various invertebrates. They are found predominantly in the intestine or body cavity of hosts. The body of adult Gregarines has a highly diverse shape: from spherical to worm-like. The body is either whole (Aseptata) or divided by an ectoplasmic septum (Cephalina) into two sections: the anterior, the protomerite, and the posterior, the deutomerite (see Figure 1, a).

The anterior end of the protomerite is usually equipped with a special outgrowth, the epimerite, which serves for attachment to the intestinal walls. For this purpose, the epimerite is often bordered by a wreath of cuticular hooks or equipped with awl-shaped appendages, etc. (see Figure 1, a). Upon reaching a certain age, usually before the beginning of sexual processes, the epimerite is shed. The body of Gregarines is covered with a thin cuticle, under which lies a layer of ectoplasm containing circular or longitudinal contractile fibrils, and according to new data, also supporting fibers. Often, a special layer of jelly is differentiated in the ectoplasm, capable of exiting outward in thin streams through special small pores of the cuticle. The outflow of jelly streams serves for the characteristic gliding movement of Gregarines, similar to the movement of diatoms. The main mass of the body is occupied by endoplasm, packed with grains of reserve nutritive

Figure 1. a-adult gregarine; b-two gregarines in a state of syzygy.

Figure 2. Two gregarines, conjugating inside a cyst; the nucleus divides into many small nuclei.

Figure 3. a-copulation of male and female gametes; b-zygote with the first nuclear division; c-formed sporocyst with 8 sporozoites inside.

substance, paraglycogen. The endoplasm contains a rounded nucleus of the vesicular type with one or several intranuclear bodies. Mouth, pharynx, digestive and contractile vacuoles are absent. The processes of nutrition, respiration, and excretion are carried out by the endosmotic route. The movement of Gregarines occurs either by the indicated gliding method or consists of sharp contractions of the body - a result of the action of muscle fibrils; many coelomic Gregarines are completely immobile. According to new data, there are small inclusions in the endoplasm of gregarines that stain strongly with osmium - a homolog of the Golgi apparatus. Many Gregarines in the adult state are connected in pairs, forming so-called syzygies (see Figure 1, b). Usually, the formation of syzygies is a harbinger of sexual reproduction. The majority of gregarines (order Eugregarinida) reproduce only by sexual means; in some forms (order Schizogregarinida) sexual reproduction alternates with asexual. The sexual process consists of conjugation, i.e., the tight adhesion of two individuals, which are then surrounded by a common cyst (see Figure 2). The nucleus of each conjugant begins to divide, giving rise to an ever-increasing number of daughter nuclei, the number of which reaches several hundred or more. These nuclei move to the periphery of each conjugant. Around each nucleus, a small section of plasma separates, and in this way, special bodies are obtained - sporoblasts. A significant part of the conjugant's plasma remains, however, unconsumed, forming a residual body. Each sporoblast turns into a sex cell, or gamete. Sometimes the gametes of both conjugants are identical, but often one of the conjugants produces immobile female gametes, and the other - mobile, flagellated male ones (see Figure 3, a). Cases have been described where both conjugants, male and female, can be distinguished from the very beginning of conjugation by peculiarities in the structure of their plasma. The gametes of both conjugants copulate in pairs and, by the fusion of their nuclei and plasma, produce zygotes, corresponding to the fertilized eggs of multicellular organisms (see Figure 3, b). Each zygote is surrounded by a dense two-layered membrane and forms a sporocyst (see Figure 3, c). These are the so-called resting stages of the cycle, serving

Figure 4. Various forms of gregarine sporocysts.

for the dissemination of parasites to other host individuals. The contents of the sporocyst, by successive division, break down into a known number (most often 8) of worm-like entities, sporozoites (see Figure 3, c). The sporocysts of Gregarines have a very diverse shape, may bear spines, hair-like appendages, etc. Due to the great constancy of shape, sporocysts play an important role in the systematics of Gregarines (see Figure 4). Cysts with sporocysts leave the host's body. Upon being swallowed by a new host individual, the cysts burst in its intestine, the sporocysts open and release the sporozoites. The latter insert their anterior end into the intestinal epithelium. The sporozoites of intestinal Gregarines remain attached to the epithelium, while those of coelomic ones penetrate the intestine and enter the body cavity. The sporozoites gradually grow, differentiate into sections (in Cephalina), and reach the adult state, then beginning sexual reproduction again. In the order Schizogregarinida, asexual reproduction is inserted into the cycle. The grown sporozoites break down into a group of entities, merozoites, and only these latter, in turn growing, begin sexual processes. The reproduction of grown sporozoites contributes to the intensification of infection within one and the same host individual. The majority of Gregarines pass the entire cycle in one host, however, in a part of Schizogregarinida, a change of hosts is observed. Thus, in Porospora, asexual reproduction takes place in the intestine of crabs, and sexual reproduction inside the gills of mollusks. The chromosomal composition of the gregarine nucleus is very interesting. According to the data of Jameson and Dobell (1918), gregarines are haploid organisms, most often with an odd number of chromosomes (3, 5, 7). Their gametes are also haploid. Only the zygotes of Gregarines possess a diploid nuclear composition. The very first division of the zygote nucleus is reductional and gives two nuclei with a haploid set of chromosomes. According to other data (Mulsow, 1911; Calkins, 1927), Gregarines in the adult state are diploid, and the reduction of the chromosome number occurs only during the division of the conjugant nuclei, immediately preceding the formation of sporoblasts. This means that only the gametes are haploid, and the copulation of the latter marks a return to the diploid state. Gregarines have no practical significance as pathogenic organisms. For the most part, they do not exert a noticeable harmful effect on hosts, although partial castration of the host has sometimes been observed under their influence. The closest relatives of Gregarines among other Sporozoa are the coccidia.